The present invention relates to a method of making silicone foam having reduced foam density and to silicone foam compositions used in such method. More particularly, the present invention relates to the use of a platinum catalyst in the form of a mixture of platinum vinyl siloxane and platinum triarylphosphine.
Prior to the present invention, elastomeric silicone foams were well-known. They have a variety of uses such as thermal and electrical insulators, flame barriers, and cushions. Even though elastomeric silicone foams are superior to organic foams in many instances, organic foams are often used based on economic consideration.
At the present time, commercially available silicone foam has a foam density which can provide a 15-25 pound/cubic ft. foam structure. One method of reducing the cost of elastomeric silicone foam, is to reduce foam density without altering the desirable physical properties of the foam material.
Although silicone foam may be made by using a blowing agent, or even mechanical mixing, another technique is by effecting the generation of hydrogen gas using a platinum catalyst as shown by Smith, U.S. Pat. No. 3,923,705. Elastomeric silicone foams can be made by forming a mixture of a silanol, a silicone hydride, and a vinyl-containing silicone in the presence of a platinum catalyst. There is effected the simultaneous generation of hydrogen gas along with cross-linking. For example, Modic, U.S. Pat. No. 4,189,545 produces silicone foam by adding water to a composition of a vinyl-containing siloxane, a hydride-containing siloxane and platinum to generate hydrogen simultaneously with cross-linking. Modic, U.S. Pat. No. 4,418,157 shows the strengthening of silicone foam by adding a resinous copolymer, while Bauman et al., U.S. Pat. No. 4,599,367 reduces silicone foam density by using a combination of silanol and water.
The present invention is based on the discovery that a substantial reduction in silicone foam density can be achieved by utilizing a platinum catalyst in the form of a mixture of a platinum vinyl siloxane complex and a platinum triaryl phosphine to effect the generation of hydrogen in a foamable composition comprising vinyl-terminated polydiorganosiloxane, a vinyl-containing organosiloxane, a hydride polysiloxane, and a hydroxylated material, all of which materials are defined more particularly below. It also has been found that the employment of a small amount, such as 0.5% to 10% by weight of the total foamable mixture, of fumed silica, treated with cyclic organosiloxane, such as octamethyl cyclotetrasiloxane, results in a significant improvement in the cell structure of the cured foam.